Too many hospital staff do not ensure older patients eat and drink properly, fail to respect their dignity and talk to them in a condescending manner, the NHS watchdog warns.

In a highly critical report the Care Quality Commission said that more than half of all hospitals in England were not meeting key standards for dignity and nutrition in elderly people, a finding it called "truly alarming and deeply disappointing". It castigated a handful of them for providing "unacceptable care".

Of 100 acute hospitals that received unannounced visits by inspectors between March and June, 45 met the NHS's standards relating to both patients' dignity and nutrition. Thirty-five did met both standards but needed to make improvements in one or both areas. And 20 – one in five – did not meet either one or both of them. Too often staff did not treat patients with kindness and compassion, it found.

Campaigners for the elderly seized on the findings – the latest evidence of poor care of older patients who are often seriously ill or physically incapacitated. "Nearly one in five hospitals completely fails to ensure that patients are eating and treated with dignity and in total nearly half of all hospitals are not doing enough," said Age UK's charity director Michelle Mitchell. "This shows shocking complacency on the part of those hospitals towards an essential part of good healthcare and there are no excuses."

At Sandwell general hospital in West Bromwich inspectors witnessed a patient who had been incontinent not being washed for 90 minutes, despite requesting help. The hospital later shut the ward concerned and replaced it with two other specialist wards.

The behaviour of staff at Alexandra hospital in Redditch, Worcestershire, prompted inspectors to decide there were major concerns about its levels of care, though improvements were then made.

And after identifying moderate concerns about nutrition and dignity at James Paget university hospitals foundation trust in Great Yarmouth, Norfolk, on a follow-up visit the commission found some patients were not receiving enough support with eating and drinking and that some who needed intravenous fluids were not getting it. The regulator issued the trust with a warning notice telling it to make urgent improvements or risk being prosecuted or having restrictions put on its operating licence.

In hospitals where essential standards were not being met inspectors found patients' call bells being put out of their reach or not responded to quickly enough, staff talking to them in a condescending or dismissive way, patients not receiving the help they needed to eat and people being interrupted during a meal and thus not finishingit.

Dame Jo Williams, the commission's chair, said: "Too often our inspectors saw the delivery of care treated as a task that needed to be completed. Those responsible for the training and development of staff, particularly in nursing, need to look long and hard at why the focus has become the unit of work rather than the person who needs to be looked after – and how this can be changed. Task-focused care is not person-centred care. Often what is needed is kindness and compassion, which cost nothing." The entire NHS needed to ensure that it made big improvements to end the scandal of poor care, she added.

Poor leadership in NHS organisations had let "unacceptable care ... become the norm", while the attitude of some staff resulted in "too many cases where patients were treated by staff in a way that stripped them of their dignity and respect", said the report.

Age UK wants the commission to undertake more spot checks and for ministers to force hospitals to publish accessible information showing rates of malnutrition on their wards.

Health secretary Andrew Lansley, who asked the commission to carry out the research, said poor care needed to be identified and stamped out.

"Everyone admitted to hospital deserves to be treated as an individual, with compassion and dignity. We must never lose sight of the fact that the most important people in the NHS are its patients. The CQC saw some exemplary care, but some hospitals were not even getting the basics right. That is simply unacceptable."

In future the planned new local HealthWatch organisations should be able to carry out their own unannounced inspections, he suggested.